Mems switches with reduced switching voltage and methods of manufacture

ABSTRACT

MEMS switches and methods of manufacturing MEMS switches is provided. The MEMS switch having at least two cantilevered electrodes having ends which overlap and which are structured and operable to contact one another upon an application of a voltage by at least one fixed electrode.

FIELD OF THE INVENTION

The invention relates to MEMS switches and methods of manufacturing MEMS switches and, more particularly, MEMS switches with reduced switching voltage and methods of manufacture.

BACKGROUND

Integrated circuit switches used in 3D and other integrated circuits can be formed from solid state structures (e.g., transistors) or passive wires (MEMS). MEMS switches are typically employed because of their almost ideal isolation, which is a critical requirement for wireless radio applications where they are used for mode switching of power amplifiers (PAs).

MEMS can be manufactured in a number of ways using a number of different tools. In general, though, the methodologies and tools are used to form small structures with dimensions in the micrometer scale. Also, many of the methodologies, i.e., technologies, employed to manufacture MEMS have been adopted from integrated circuit (IC) technology. For example, almost all MEMS are built on wafers and are realized in thin films of materials patterned by photolithographic processes. More specifically, the fabrication of MEMS use three basic building blocks: (i) deposition of thin films of material on a substrate, (ii) applying a patterned mask on top of the films by photolithographic imaging, and (iii) etching the films selectively to the mask.

Depending on the particular application and engineering criteria, MEMS structures can come in many different forms. For example, MEMS can be realized in the form of a single cantilever structure such as, for example, shown in U.S. Pat. No. 7,265,492. In this cantilever application, a single cantilever arm (suspended electrode) is pulled toward a fixed electrode by application of a voltage. In known applications, the voltage required to pull the suspended electrode down to the fixed electrode by electrostatic force may be high, which has been seen to cause unwanted charging on insulator after prolonged use and eventual failure of the switch. In certain applications, the high voltage, e.g., 100 volts, is also difficult to obtain since this has to be stepped up from about 1.5 volts to about 5 volts. The minimum voltage required is called pull-in voltage, which is dependent on area of the electrode, spacing or gap between the suspended and fixed electrodes, and spring constant of the membrane or springs.

Lowering the pull-in voltage without decreasing the gap and without softening the spring is desirable, as the spring provides the restoring force and determines the switching speed. In U.S. Pat. No. 7,265,492, a pair of side parallel-plate electrostatic actuators is implemented for lowering or eliminating of the bias voltages. These additional electrostatic actuators are used to reduce or eliminate the bias voltage to be applied on the fixed signal electrode. In implementation, the fixed electrode of the side parallel-plate electrostatic actuators can be elevated above a fixed signal electrode. Thus due to a smaller gap, the pull-in voltage required to pull the suspended electrode down to the fixed electrode can be lowered. However, the MEMS shown in U.S. Pat. No. 7,265,492 are not hermetically sealed, and the additional electrostatic actuators can increase fabrication costs.

Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.

SUMMARY

In a first aspect of the invention, a structure comprises at least two cantilevered electrodes having ends which overlap and which are structured and operable to contact one another upon an application of a voltage by at least one fixed electrode.

In a second aspect of the invention, a method of fabricating a switch comprises forming at least two cantilever electrodes and at least one fixed electrode through a series of resist deposition and patterning steps.

In yet another aspect of the invention, a method of forming a switch comprises: depositing layers of resist on a structure; patterning the resist to form sequential openings; sequentially depositing metal or metal alloy within the sequential openings until at least two cantilever electrodes and at least one voltage applying electrode are formed within the layers of resist; depositing a liner over an uppermost layer of the layers of resist; forming openings in the liner; etching the layers of the resist through the opening until the cantilever electrodes and the at least one voltage applying electrode are in a void; and sealing the void with additional liner material to form a hermetically sealed dome.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which:

FIGS. 1 a-1 i show intermediate structures and respective fabrication processes in accordance with the invention;

FIG. 2 shows a MEMS structure in accordance with a first aspect of the invention;

FIG. 3 shows a MEMS structure in accordance with a second aspect of the invention;

FIG. 4 shows a MEMS structure in accordance with a third aspect of the invention;

FIG. 5 shows a MEMS structure in accordance with a fourth aspect of the invention; and

FIG. 6 shows a MEMS structure in accordance with a fifth aspect of the invention.

DETAILED DESCRIPTION

The invention relates to MEMS switches and methods of manufacturing MEMS switches and, more particularly, MEMS switches with reduced switching voltage and methods of manufacture. In implementation, the invention includes methods and structures of several novel MEMS switches optimized for (1) switching voltage (i.e. reducing it) and (2) reliability.

The MEMS switches of the invention include at least a double cantilever arrangement hermetically sealed within a nitride type liner, for example. In operation, a gap between the electrodes is reduced, compared to conventional MEMS switches. This arrangement will reduce the minimum switching voltage required to pull the electrodes together (i.e., reduced pull-in voltage) and/or the on time of the voltage. Accordingly, unwanted charging on insulator and failure of the switch can be reduced, compared to known conventional switches. Also, in operation, the MEMS switches substantially eliminate arcing, as well as large dielectric breakdown attributable to higher switching voltages. Although the MEMS switches are shown with a nitride hermetic seal, MEMS switches fabricated using the same methodology, either without nitride hermetic seals or with other methods of hermetic seals, such as a MEMS switch inside a cavity with a bonded chip or wafer cap, are contemplated by the present invention.

Exemplary Fabrication Processes in Accordance with the Invention

FIGS. 1 a-1 i show intermediate structures and respective fabrication processes in accordance with the invention. More specifically, the processes shown and described with reference to FIGS. 1 a-1 h are directed to the embodiment of FIG. 2. However, it should be recognized by those of skill in the art that with some modifications and/or additions to the processes described herein, e.g., patterning, metallization and/or deposition processes, the processes of FIGS. 1 a-1 i can be used to fabricate any of the embodiments described herein. Although such modifications and/or additions should become obvious to those of skill in the art after an explanation of each of the embodiments, some further explanation of the additional and/or modified processes are described herein as necessary for a more thorough understanding of the invention.

More specifically, FIG. 1 a shows a beginning structure in accordance with the invention. The beginning structure includes a dielectric material 10 with a plurality of vias 12. As should be understood by those of skill in the art, the dielectric material 10 may be an M+1 wiring layer in an integrated circuit. Although not shown in FIG. 1, it should be understood that the dielectric material 10 may be provided on a wafer of any known type used with the formation of integrated circuits. For example, the wafer can be silicon, BULK, SOI, SiGe; quartz; glass; or Gallium arsenide, to name a few. The vias 12 can be metallized using any combination of methods known in the art, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), electroplated deposition (ECP), metal-organo chemical vapor deposition (MOCVD), etc. In one exemplary embodiment, the vias are tungsten plugs, with TiN liners. In another embodiment, the vias are formed using copper with TaN/Ta liners. In another embodiment, the vias are ‘tapered vias’ which are metallized with the conductor layer used to form 16 a and 18 a shown in FIG. 1 b.

The plurality of vias 12 are formed using conventional lithographic processes. For example, a resist is deposited on the dielectric material 10 and selective portions of the resist are exposed to form openings. In subsequent processes, the dielectric material 10 is etched using a conventional process such as, for example, reactive ion etching (RIE) to form vias. The vias are filled with known metals or metal alloys to form the vias 12. The resist can be stripped away. The vias 12 can act as conductive pads as noted in more detail below.

In FIG. 1 b, the lower conductive MEMS switch electrodes are formed. These can be formed using any known method, such as by depositing the conductor, lithographically patterning it, etching it, and removing the photoresist used for lithographic patterning. Alternatively, other known methods, such as lift-off or damascene could be used. FIG. 1 b shows a damascene method in which a sacrificial resist layer 14 is deposited over the structure of FIG. 1 a. In one embodiment, the sacrificial resist layer 14 comprises Polymethylglutarimide (PMGI). PMGI is compatible with most g-line, i-line, and DUV photoresists and has excellent adhesion to Si, SiN, NiFe, Cu, Au, GaAs, and other III-V/III-VI materials. PMGI also exhibits a high thermal stability and can be applied in any conventional manner such as, for example, spin-coating. The PMGI can be stripped in NMP and DMSO-based removers. Also, PMGI is DUV, E-beam, and x-ray sensitivity, as well as exhibits a high etch rate in oxygen plasma.

In conventional processes, the sacrificial resist layer 14 is patterned to form openings. The openings are filled with a metal such as gold; although, other metals or metal alloys are also contemplated by the invention such as AlCu, W, or Cu. Prior to the deposition of the metal, one or more refractory metals, such as Ti, TiN, Ta, TaN, Ru, etc. can be used to line the vias. In the embodiment described herein, the metal will form fixed electrodes 16 a and 16 b and cantilevered electrodes 18 a and 18 b.

In processing steps shown in FIGS. 1 c-1 f, additional deposition and patterning processes are sequentially shown to build the fixed electrodes 16 a and 16 b and cantilevered electrodes 18 a and 18 b. The deposition and patterning processes are similar to that described with reference to FIG. 1 b and, as such, additional explanation is not required for a complete understanding of the invention. In embodiments, the processes described herein result in the arm (beam) of the cantilever electrode 18 b being formed in the processes shown in FIG. 1 d and the arm of the cantilever electrode 18 a being formed in the processes shown in FIG. 1 f. Also, as shown in FIG. 1 f, the electrodes 18 a and 18 b have respective end portions 18 a ₁ and 18 b ₂ that overlap. The overlapping end portions 18 a ₁ and 18 b ₂, upon application of a voltage, will close the switch, as discussed in greater detail below.

As shown in FIG. 1 g, a hermetic dielectric, such as Si₃N₄ (nitride) liner 20 is deposited over the structure of FIG. 1 h. In embodiments, the liner 20 can be SiN. In FIG. 1 h, holes 22 are etched into the liner 20 to form openings, exposing the sacrificial resist. In subsequent processes, a wet etching using, for example, NMP (N-methylpyrrolidone) is used to dissolve the sacrificial resist encapsulated within the liner 20, creating a void 24. The void 24 is hermetically sealed by a deposition of Nitride 23 in order to close the holes 22 and form a hermetically sealed dome, as shown in FIG. 1 i. As discussed above, the nitride dome is used to hermetically seal the MEMS switch and is optional. Although the method used for forming the freestanding cantilever beams uses the sacrificial PMGI resist, any known method of forming cantilever beams, such as using sacrificial α-silicon, subtractive-etch wiring, tapered via wiring, etc. could also be used.

FIRST ASPECT OF THE INVENTION

FIG. 2 shows a MEMS structure in accordance with a first aspect of the invention. In this aspect of the invention, the electrodes 16 a and 16 b and 18 a and 18 b are hermetically sealed within the nitride layer 25. The nitride dome 25 can be oval shaped, as shown in FIG. 2, rectangular shaped, as shown in FIG. 1, or any other known shape. In embodiments, the arms of the cantilever electrodes 18 a and 18 b are about 50 microns long, 9 microns high and 20 microns long; although other dimensions are also contemplated by the invention. Also, in further embodiments, a distance “X” between the respective overlapping end portions 18 a ₁ and 18 b ₂ is about two microns; although, other distances are also contemplated by the invention. To decrease the distance “X”, vertically extending portions (as shown in the embodiment of FIG. 5) can extend from each of the cantilever electrodes 18 a and 18 b.

In operation, upon the application of a positive voltage to the fixed electrode 16 a, the cantilever electrode 18 a will be pulled down towards the cantilever electrode 18 b. Also, upon a negative voltage applied to the fixed electrode 16 b, the cantilever electrode 18 b will be pushed up towards the cantilever electrode 18 a. At a predetermined designed voltage, the respective overlapping end portions 18 a ₁ and 18 b ₂ will make contact with one another, i.e., travel the distance “X”, thereby closing the switch. In the off state (0 voltage), the cantilever electrodes 18 a and 18 b will return to their original position, with a space “X” between the respective ends.

In one design, the voltage applied to the fixed electrode 16 a is about 30 volts and the fixed electrode 16 b is about −30 volts. This design voltage can be significantly lower than known conventional systems as the two cantilever arms are each designed and arranged to move a smaller distance than a single arm in a conventional system. More specifically, there can be a reduced switching voltage due to bending of both arms and the use of two voltage electrodes (e.g., 16 a and 16 b).

This reduced voltage is a minimum switching voltage required to pull the electrodes together (i.e., pull-in voltage). This reduced voltage can result in many advantages such as, for example, reduced on time voltage, unwanted charging on insulator and reduced failure of the switch (compared to known conventional switches). Also, in operation, the MEMS switch of FIG. 2 substantially eliminates arcing, as well as large dielectric breakdown attributable to higher switching voltages.

SECOND ASPECT OF THE INVENTION

FIG. 3 shows a MEMS structure in accordance with a second aspect of the invention. In this aspect of the invention, the fixed electrode 16 b has been eliminated. As in the other embodiments, the fixed electrode 16 a and the cantilever electrodes 18 a and 18 b are hermetically sealed within the nitride layer 22. In embodiments, the arms of the cantilever electrodes 18 a and 18 b are about 50 microns long, 9 microns high and 20 microns long; although other dimensions are also contemplated by the invention. Also, in further embodiments, a distance “X” between the overlapping respective end portions 18 a ₁ and 18 b ₁ is about two microns; although, other distances are also contemplated by the invention. To decrease the distance “X”, vertically extending portions (as shown in the embodiment of FIG. 5) can extend from one or both of the cantilever electrodes 18 a and 18 b.

In operation, upon the application of a positive voltage to the fixed electrode 16 a, the cantilever electrode 18 a will be pulled down towards the cantilever electrode 18 b. At a predetermined designed voltage, the respective overlapping end portions 18 a ₁ and 18 b ₁ will make contact with one another, i.e., travel the distance “X”, thereby closing the switch. In one design, the voltage applied to the fixed electrode 16 a is about 100 volts. In the off state (0 voltage), the cantilever electrode 18 a will return to its original position, with a space “X” between the respective end portions 18 a ₁ and 18 b ₂. In this embodiment, the cantilever electrode 18 b is designed to remain stationary.

This arrangement also provides advantages such as, for example, reduced sticktion of the electrodes 18 a and 18 b. More specifically, as there are two cantilever arms 18 a and 18 b, it is theorized that that switch will stayed in the closed position, upon the application of a voltage, better than conventional MEMS switches. This will ensure that the switch will not fail.

THIRD ASPECT OF THE INVENTION

FIG. 4 shows a MEMS structure in accordance with a third aspect of the invention. In this aspect of the invention, the electrodes 16 a and 16 b and 18 a and 18 b are hermetically sealed within the nitride layer 22. In embodiments, the arms of the cantilever electrodes 18 a and 18 b have different lengths, such that 18 b does not extent over 16 a, where the arm of the cantilever electrode 18 a is longer than the arm of the cantilever electrode 18 b (although this can be reversed). To decrease the distance “X”, vertically extending portions (as shown in the embodiment of FIG. 5) can extend from one or both of the cantilever electrodes 18 a and 18 b.

In embodiments, the arm of the cantilever electrode 18 a extends over both of the fixed electrodes 16 a and 16 b. Also, in the embodiment of FIG. 4, the cantilever electrode 18 b is fixed, i.e., embedded, to the nitride layer 22. This can be achieved by depositing the nitride layer 22 directly onto an upper surface of the cantilever electrode 18 b. In an alternative embodiment, the cantilever electrode 18 b can also be floating by adding an upper layer of sacrificial material (PMGI) prior to the deposition of the nitride layer 22. Also, in further embodiments, a distance “X” between the overlapping cantilevers 18 a and 18 b is about two microns; although, other distances are also contemplated by the invention. To decrease the distance “X”, vertically extending portions (as shown in the embodiment of FIG. 5) can extend from the cantilever electrode 18 a or the fixed electrode 16 b.

In operation, upon the application of a positive voltage to the fixed electrode 16 a, the cantilever electrode 18 a will be pulled down towards the fixed electrode 16 b. Also, upon a negative voltage applied to the cantilever electrode 18 b, the cantilever electrode 18 a will be pushed down towards the fixed electrode 16 b. At a predetermined designed voltage, the end portion 18 a ₁ will make contact with the fixed electrode 16 b, i.e., travel the distance “Y”, thereby closing the switch. In the off state (0 voltage), the cantilever electrode 18 a will return to its original position, with a space “Y” between the cantilever electrode 18 a and the fixed electrode 16 b. In this design, the cantilever electrode 18 b remains stationary, as it is fixed to the nitride liner 22.

In one design, the voltage applied to the fixed electrode 16 a is about 50 volts and the voltage applied to the cantilever electrode 18 b is about −50 volts. This design voltage can be significantly lower than known conventional systems as the arm of the cantilever electrode 18 a is being pushed and pulled by the use of two voltage electrodes (e.g., 16 a and 18 b).

This reduced voltage is a minimum switching voltage required to pull the electrodes together (i.e., pull-in voltage). This reduced voltage can result in many advantages such as, for example, reduced on time voltage, unwanted charging on insulator and reduced failure of the switch (compared to known conventional switches). Also, in operation, the MEMS switch of FIG. 4 substantially eliminates arcing, as well as large dielectric breakdown attributable to higher switching voltages.

FOURTH ASPECT OF THE INVENTION

FIG. 5 shows a MEMS structure in accordance with a fourth aspect of the invention. In this aspect of the invention, an additional cantilever electrode 18 c is formed using the processes described above, e.g., adding an additional deposition layer of resist and additional patterning steps. The electrodes 16 a, 16 b and 18 a-18 c are hermetically sealed within the nitride layer 22. In embodiments, the arm of the cantilever electrode 18 a also includes a vertical extending protrusion (nub) 18 a ₂. As in other embodiments, the cantilever electrodes 18 a and 18 b are about 50 microns long, 9 microns high and 20 microns long; although other dimensions are also contemplated by the invention. Also, in embodiments, a distance “X” between the respective portions 18 a ₂ and 18 b ₁ is about two microns; although, other distances are also contemplated by the invention. To decrease the distance “X”, vertically extending portions can extend from each of the cantilever electrodes 18 a and 18 b.

In operation, upon the application of a positive voltage to the fixed electrode 16 b, the cantilever electrode 18 b will be pulled down towards the cantilever electrode 18 a. Also, upon a negative voltage applied to the fixed electrode 16 a and a positive voltage applied to the cantilever electrode 18 c, the cantilever electrode 18 a will be pushed upwards toward the cantilever electrode 18 b. At a predetermined designed voltage, the respective nub portion 18 a ₂ and the end portion 18 b ₁ will make contact with one another, i.e., travel the distance “X”, thereby closing the switch. In the off state (0 voltage), the cantilever electrodes 18 a and 18 b will return to their original position, with a space “X” between the respective ends.

In one design, the voltage applied to the fixed electrode 16 b and the cantilever electrode 18 c is about 30 volts. Also, the voltage applied to the fixed electrode 16 a is about −30 volts. This design voltage can be significantly lower than known conventional systems as the two cantilever arms (18 a and 18 b) are each designed and arranged to move a smaller distance than a single arm in a conventional system. More specifically, there can be a reduced switching voltage due to bending of both arms and the use of three voltage electrodes (e.g., 16 a, 16 b and 18 c).

This reduced voltage is a minimum switching voltage required to pull the electrodes together (i.e., pull-in voltage). This reduced voltage can result in many advantages such as, for example, reduced on time voltage, unwanted charging on insulator and reduced failure of the switch (compared to known conventional switches). Also, in operation, the MEMS switch of FIG. 5 substantially eliminates arcing, as well as large dielectric breakdown attributable to higher switching voltages.

FIFTH ASPECT OF THE INVENTION

FIG. 6 shows a MEMS structure in accordance with a fifth aspect of the invention. In this aspect of the invention, an additional cantilever electrode 18 c is formed using the processes described above, e.g., adding an additional deposition layer of resist and additional patterning steps. Also, the fixed electrode 16 b can be raised to be closer to the cantilever electrode 18 b which, in turn, provides an improved response time (as the space between the fixed electrode 16 b and the cantilever electrode 18 b is closed). The height of the fixed electrode 16 b is higher than the height of the fixed electrode 16 a. To decrease the distance “X”, a vertically extending portion can also extend from the cantilever electrodes 18 b. A gap is approximately 5 to 10 microns between the. electrode 16 b and the cantilever electrode 18 b. In embodiments, the electrodes 16 a, 16 b and 18 a-18 c are hermetically sealed within the nitride layer 22.

In embodiments, the arm of the cantilever electrode 18 a also includes a vertical extending protrusion (nub) 18 a ₂. As in other embodiments, the cantilever electrodes 18 a and 18 b are about 50 microns long, 9 microns high and 20 microns long; although other dimensions are also contemplated by the invention. Also, in further embodiments, a distance “X” between the respective end portions 18 a ₁ and 18 b ₂ that overlap is about two microns; although, other distances are also contemplated by the invention.

In operation, upon the application of a positive voltage to the fixed electrode 16 b, the cantilever electrode 18 b will be pulled down towards the cantilever electrode 18 a. Also, upon a negative voltage applied to the fixed electrode 16 a and a positive voltage applied to the cantilever electrode 18 c, the cantilever electrode 18 a will be pushed upwards toward the cantilever electrode 18 b. At a predetermined designed voltage, the respective nub portion 18 a ₂ and the end portion 18 b ₁ will make contact with one another, i.e., travel the distance “X”, thereby closing the switch. In the off state (0 voltage), the cantilever electrodes 18 a and 18 b will return to their original position, with a space “X” between the respective ends.

In one design, the voltage applied to the fixed electrode 16 b and the cantilever electrode 18 c is about 30 volts. Also, the voltage applied to the fixed electrode 16 a is about −30 volts. This design voltage can be significantly lower than known conventional systems as the two cantilever arms (18 a and 18 b) are each designed and arranged to move a smaller distance, than a single arm in a conventional system. More specifically, there can be a reduced switching voltage due to bending of both arms and the use of three voltage electrodes (e.g., 16 a, 16 b and 18 c).

This reduced voltage is a minimum switching voltage required to pull the electrodes together (i.e., pull-in voltage). This reduced voltage can result in many advantages such as, for example, reduced on time voltage, unwanted charging on insulator and reduced failure of the switch (compared to known conventional switches). Also, in operation, the MEMS switch of FIG. 6 substantially eliminates arcing, as well as large dielectric breakdown attributable to higher switching voltages.

The structures as described above are used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

While the invention has been described in terms of exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modifications and in the spirit and scope of the appended claims. 

It is claimed:
 1. A method of fabricating a switch comprising: forming a first fixed electrode and a second fixed electrode; forming a first cantilevered electrode aligned vertically over the first fixed electrode and the second fixed electrode, and operable to directly contact the second fixed electrode upon an application of a voltage to the first fixed electrode; forming a second cantilevered electrode aligned vertically over the second fixed electrode, and which has an end that overlaps the first cantilevered electrode; and forming a hermetically sealed volume encapsulating the first fixed electrode, the second fixed electrode, the first cantilevered electrode, and the second cantilevered electrode.
 2. The method of claim 1, wherein the forming the hermetically sealed volume comprises: forming at least one hole in a nitride liner to gain access to a sacrificial resist; etching the sacrificial resist; and closing the at least one hole by deposition of additional liner material.
 3. The method of claim 1, wherein the hermetically sealed volume is a dome.
 4. The method of claim 3, wherein the dome is oval shaped.
 5. The method of claim 1, wherein overlapping portions of the first cantilevered electrode and the second cantilevered electrode are separated by a vertical distance of about two microns.
 6. The method of claim 1, wherein the second cantilevered electrode is formed above the first cantilevered electrode.
 7. The method of claim 1, wherein the first cantilevered electrode is formed such that upon application of a voltage to the second cantilevered electrode, the first cantilevered electrode is operable to directly contact the second fixed electrode. 